As-made Materials Research Articles

Morphologies and dispersion stabilities of carbon-encapsulated metal (Ag and Cu) nanoparticles synthesized by pulsed-wire evaporation (PWE)

Carbon-encapsulated Ag and Cu nanoparticles were synthesized using pulsed-wire evaporation (PWE). All as-made materials were composed of nanocapsules with a uniform particle size at and below 50 nm. The nanocapsules consisted of an outer carbon layer shaped multi-shell. The dispersion stability of the solvent depends on the measured zeta potential for the particles in water, ethanol, ethylene glycol (EG), and polyethylene glycol (PEG). In the case of Ag@C, a stable dispersion was observed in the PEG suspension only. The stable dispersions of Cu@C were observed in EG and PEG without showing any clarification or sedimentation. Flocculation by a coalescing reaction between the nanoparticles was insignificant due to carbon layers on the surfaces of the metal (Ag and Cu) particles.

A Multinuclear Solid-State NMR Study of Templated and Calcined Chabazite-Type GaPO-34

The open-framework gallophosphate GaPO-34 is prepared with either 1-methylimidazole or pyridine as the structure-directing agent. 13C and 1H NMR spectra for these two variants of the as-made GaPO-34 are fully assigned, confirming the presence of the protonated amine and water within the pores of both materials. 31P MAS NMR confirms the presence of three crystallographic P sites, while 71Ga MAS and MQMAS NMR spectra reveal three crystallographic Ga sites: two tetrahedral and one six-coordinate. Simulations of 69Ga MAS NMR spectra from these results are in good agreement with spectra acquired at B0 = 20.0 T, and assignments are supported by first-principles calculations. 19F MAS NMR proves the presence of Ga-bridging fluoride within the as-made materials, leading to the six-coordinate gallium. Calcination removes the organic species and fluoride, yielding a microporous chabazite-type GaPO4, containing one tetrahedral Ga site. Exposure to moist air yields calcined, rehydrated GaPO-34 containing four-, five-, and six-coordinate gallium. Upon heating this material, loss of crystallinity is observed by powder X-ray diffraction and NMR, with the latter revealing a range of P and Ga environments. The thermal instability of calcined, rehydrated GaPO-34 contrasts with the isomorphous aluminophosphate, showing that apparently analogous materials may have important differences in reactivity.

Steam-assisted crystallization of TPA+-exchanged MCM-41 type mesoporous materials with thick pore walls

Hierarchical MCM-41/MFI composites were synthesized through ion-exchange of as-made MCM-41 type mesoporous materials with tetrapropylammonium bromide and subsequent steam-assisted recrystallization. The obtained samples were characterized by powder X-ray diffraction (XRD), UV–vis diffuse reflectance spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis, FT-IR, 1H–13C CP/MAS and nitrogen adsorption–desorption. The XRD patterns show that the MCM-41/MFI composite possesses both ordered MCM-41 phase and zeolite MFI phase. SEM and TEM images indicate that the recrystallized materials retained the mesoporous characteristics and the morphology of as-made mesoporous materials without the formation of bulky zeolite, quite different from the mechanical mixture of MCM-41 and MFI structured zeolite. Among others, lower recrystallization temperature and the introduction of the titanium to the parent materials are beneficial to preserve the mesoporous structure during the recrystallization process.

Electrochemical and Physical Properties of Ti-Substituted Layered Nickel Manganese Cobalt Oxide (NMC) Cathode Materials

The effect of partial substitution of titanium for cobalt on 3b sites in Li1+z(NixMnxCo1-2x-yTiy)O2 (x = 1/3, 0.4) cathode materials (NMCs) has been investigated for this study. Materials were prepared by glycine-nitrate combustion and co-precipitation methods and solid solutions were observed over a narrow substitution range limited to below about y = 0.07 for both the x = 1/3 and x = 0.4 series, although this depended on the synthesis technique. Increased discharge capacities, lower first cycle inefficiencies and better cycling behavior were observed for several of the Ti-substituted compositions in electrochemical cells. The improved first cycle efficiencies allow higher discharge capacities to be obtained between normal voltage limits, which partly explain the observations. Results from synchrotron X-ray diffraction also indicate that Ti substitution leads to a decreased cell volume change during deintercalation of lithium, which contributes to the improved cycling performance. Magnetic measurements suggest that reduction of some Mn4+ to Mn3+ compensates for the aliovalent substitution in as-made lithium-stoichiometric materials.

Structural, Optical, and Magnetic Properties of Highly Ordered Mesoporous MCr2O4 and MCr2–xFexO4 (M = Co, Zn) Spinel Thin Films with Uniform 15 nm Diameter Pores and Tunable Nanocrystalline Domain Sizes

Herein is reported the synthesis and characterization of nanocrystalline cobalt chromite (CoCr2O4) and zinc chromite (ZnCr2O4) thin films with highly ordered cubic networks of open pores averaging 15 nm in diameter. We also show that the synthesis method employed in this work is readily extendable to solid solutions of the type MCr2–xFexO4 (M = Co, Zn), which could pave the way for innovative device design. All of these materials can be prepared by facile coassembly of hydrated nitrate salts with an amphiphilic diblock copolymer, referred to as KLE. The as-made materials are amorphous thin films with face-centered-cubic close-packed pore structures. Electron microscopy, X-ray diffraction, grazing incidence small-angle X-ray scattering, krypton physisorption, UV–vis spectroscopy, time-of-flight secondary ion mass spectrometry, X-ray photoelectron spectroscopy, and Raman spectroscopy studies collectively verify that both the transition metal chromites and the solid solutions are well-defined at the nanoscale and the microscale. In addition, the data show that the different thin film materials are nanocrystalline after annealing in air at 600 °C, adopt the spinel structure in phase-pure form, and that the conversion of the initially amorphous frameworks comes at little cost to the ordering of the cubic pore-solid architectures. Magnetization studies as a function of temperature and field further reveal the high quality of the KLE-templated CoCr2O4 thin films with both long-range ferrimagnetic order and spiral magnetic order at low temperatures, in agreement with previous findings.

Tetrahedral Atom Ordering in a Zeolite Framework: A Key Factor Affecting Its Physicochemical Properties

Three gallosilicate natrolites with closely similar chemical composition but differing in the distribution of Si and Ga over crystallographically different tetrahedral sites (T-sites) show striking differences in their cation exchange performance. The ability to exchange Na(+) by the larger alkali metal cations decreases upon increasing the size of the cation, as expected, but also with the degree of T-atom ordering. To seek an insight into this phenomenon, the crystal structures of 11 different zeolites, which show variations in degree of T-atom ordering, nature of countercation, and hydration state, have been refined using synchrotron diffraction data. While the three as-made sodium materials were characterized to have a low, medium, and high degree of ordering, respectively, their pore sizes are close to the size of the bare Na(+) cation and much smaller than that of the larger alkali cations, which are nonetheless exchanged into the materials, each one at a different level. Interestingly, large differences are also manifested when the Na(+) back-exchange is performed on the dehydrated K(+) forms, with crystallographic pore sizes too small even to allow the passage of Na(+). Although the thermodynamic data point to small differences in the enthalpy of the Na(+)/K(+) exchange in the three materials, comparison of the "static" crystallographic pore sizes and the diameter of the exchanged cations lead us to conclude that during the exchange process these zeolites undergo significant deformations that dynamically open the pores, allowing cation traffic even for Cs(+) in the case of the most disordered material. In addition to the very large topological flexibility typical of the natrolite framework, we propose as a hypothesis that there is an additional flexibility mechanism that decreases the rigidity of the natrolite chain itself and is dependent on preferential siting of Si or Ga on crystallographically different T-sites.

Direct Hydrothermal Synthesis and Physical Properties of Rare-Earth and Yttrium Orthochromite Perovskites

We describe a systematic study of the hydrothermal synthesis of rare-earth orthochromites, RCrO3 (R = La, Pr, Sm, Gd, Dy, Ho, Yb, and Lu) and YCrO3. All nine of these materials can be prepared in a single step by hydrothermal treatment of an amorphous mixed-metal hydroxide at temperatures above 300 °C upon heating around 24 h, with no post-synthesis annealing needed. The as-made materials are highly crystalline powders with submicrometer particle size. In the case of LaCrO3 the addition of solution additives to the hydrothermal synthesis allows some modification of crystallite form of the material, and in the presence of sodium dodecylsulfate nanocrystalline powders are produced. Profile refinement of powder X-ray diffraction (XRD) data show that each of the RCrO3 materials adopts an orthorhombic distorted (Pbmn) perovskite structure. Detailed, magnetization studies as a function of temperature reveal the high quality of the specimens, with low temperature antiferromagnetic behavior seen by direct current (DC) magnetization, and TN values that correlate with the structural distortion. A high temperature structural phase transition (∼ 540 K to rhombohedral R3̅c) seen by variable temperature XRD in the hydrothermally prepared LaCrO3 is consistent with the reported behavior of the same composition prepared using conventional high temperature synthesis.

In SituTransformation of TON Silica Zeolite into the Less Dense ITW: Structure-Direction Overcoming Framework Instability in the Synthesis of SiO2Zeolites

Under specific synthesis conditions the crystallization of a dense silica zeolite (TON) is followed by its in situ transformation into a less dense and, in the absence of occluded species, less stable zeolite (ITW). Periodic ab initio calculations including energy corrections for van der Waals interactions as well as zero-point and thermal effects are used first to assess the relative stability of both SiO(2) (calcined) phases and then to investigate host-guest interactions in the as-made zeolites, as well as their relative stability. The less dense SiO(2)-ITW is less stable than SiO(2)-TON, with an energy difference that is significantly larger than expected from their difference in molar volume. This extra destabilization is ascribed to the strained double 4-ring units of silica tetrahedra (D4R). Regarding the as-made materials, the organic cation fills in more efficiently the zeolitic voids in ITW than in TON, bringing about a larger stabilization in the former owing to the extension of the long-range addition of dispersion force contributions. On the other hand, fluoride induces a polarization of the silica framework that is highly localized in TON (showing pentacoordinated [SiO(4/2)F](-) units) but has a large global character in ITW (where fluoride is encapsulated into D4R units). We argue that the structure-directing role toward D4R materials that has been proposed for fluoride consists fundamentally in the ability to induce a global polarization of the silica framework that allows relaxation of the strain associated with these units. In this sense, fluoride stabilizes the otherwise strained D4R-SiO(2) frameworks making them reachable for crystallization. This work documents a case in which the structure directing agents "choose" a structure not kinetically but through stabilization.

Anion-Exchange Induced Phase Transformation of Mesostructured Silica

Mesostructured silica synthesized using cetyltriethylammonium (CTEA) ion as the pore-directing agent in acidic environments was ready to have interfacial anions exchanged at ambient temperature. In situ techniques including small-angle X-ray scattering (SAXS) and optical microscopy were used to examine the structural and morphological changes of the as-made mesostructured materials, and pyrene fluorescence quenching experiment was used to probe the variation in interfacial environment during the anion-exchange processes. About one-half of the interfacial anions between the silica framework and micelle were exchanged based on the fluorescence quenching experiments of pyrene, which was dissolved in the hydrophobic core of the micelle. Altering in the pore structure and the architecture of the silica framework was observed when the interfacial anions were exchanged by anions of higher salting-in power. The resultant phase transformation and morphological change were always toward pore structures of lower curvature. The influence of anion follows the Hofmeister series commonly encountered in macromolecular and biological systems. The mesophase transformation induced by the interfacial anion exchange provides a unique technique to study anions which are not yet included in the Hofmeister series.

Reactive pore expansion during ammonia vapor post-treatment of ordered mesoporous silica prepared with mixed glucopyranoside and cationic surfactants

n-Octyl-β- d-glucopyranoside (C 8G 1) has the potential to serve as a functional, nonionic template for mesoporous oxides, but its use has been limited due to its propensity to form lamellar mesostructures. Here, 2D hexagonal-structured mesoporous silicas with variable pore sizes are synthesized with C 8G 1 by the “nanocasting” technique (polycondensation of tetramethoxysilane in concentrated acidic solutions with evaporation) by mixing it with cetyltrimethylammonium bromide (CTAB) to promote ordering. In an effort to stabilize the silica network structure, a mild treatment with ammonia vapor at 50 °C is introduced. Without ammonia treatment, the pore diameter remains constant as the C 8G 1:CTAB weight ratio increases even up to 0.3:1. When the as-made materials are treated with ammonia vapor, the degree of long-range order of the samples increases, and pore expansion is observed which increases in magnitude as the C 8G 1 content increases. Large degrees of expansion increase pore size fluctuations but do not eliminate long-range order. Materials with well-ordered, uniform pores can be made with C 8G 1/CTAB mixtures by limiting the amount of ammonia or increasing the amount of silica precursor. A mechanistic study suggests that micelle swelling occurs with CTAB, but is limited by increased polycondensation of silica and favorable headgroup–silica interactions. With the glucopyranoside surfactant, the extent of expansion is greater not only because of a change in the physical interaction with silica, but also because the Maillard reaction occurs under surprisingly mild conditions.

Influence of Synthesis Methods on Tungsten Dispersion, Structural Deformation, and Surface Acidity in Binary WO3−ZrO2 System

WO3-ZrO2 catalysts were synthesized by precipitating the aqueous solutions of zirconium oxynitrate and ammonium metatungstate with ammonium hydroxide. The white slurry precipitate was treated under three different conditions. In the as-made materials, the amorphous phase was formed in the aged and refluxed samples, while well-crystallized tetragonal and monoclinic phases were obtained in the hydrothermally treated sample. The real amount of tungsten loaded in the samples was similar for the three samples, independently of the treatments; however, the tungsten surface atomic density in the annealed WO3-ZrO2 samples varied between 6 and 9 W atoms/nm2. Two different contrast types of aggregates were determined by scanning electron microscopy, the white particles which are rich in W, and the gray ones which are rich in zirconium; both of them were formed in the calcined solids prepared under aging or reflux condition. A very high dispersion of tungsten species on the zirconia surface was achieved in the hydrothermally treated sample. The degree of the interaction between WO(x) and ZrO2 surface strongly modified the Zr-O bond lengths and bond angles in the structure of tetragonal zirconia as proved by X-ray diffraction analysis and the Rietveld refinement. The catalyst obtained under hydrothermal condition exhibited the highest dispersion of tungsten species in the zirconia, which in turn causes strong structural deformation of the tetragonal ZrO2 phase responsible of the strongest surface acidity and, consequently, the optimum catalytic activity for n-hexane isomerization.

Alkali-containing mesoporous (alumino) silicate materials with very high metal loading

Mesoporous aluminosilicates with Si/Al ratios ranging from approximately 1 to ∞ (for a pure-silica compound) have been synthesized in the presence of sodium and cetyl trimethyl ammonium (CTMA +) cations. All materials are thermally stable and retain their highly ordered hexagonal structure upon removal of surfactant molecules at 800 K. Original procedures have been explored to exchange selectively surfactant and/or sodium cations from as-made materials. Treatments in water exchange exclusively sodium ions, without extracting surfactant molecules, whilst treatments in ethanol remove preferentially organic molecules. Combining these two procedures allowed us to exchange completely CTMA + and Na + cations and to prepare highly ordered Li +, K +, Cs + and Ca 2+-modified materials with very high alkali metal concentrations. The stability of the exchanged mesoporous compounds depends on the aluminum content and on the nature of the alkali cation. All materials are particularly stable upon exchange with Li + and Ca 2+ cations. By contrast, the stability of solids exchanged with Cs + or K + cations decreases with decreasing the aluminum content. In the particular case of pure silica MCM-41, the hexagonal structure is retained upon exchange of surfactant molecules with Li + and Ca 2+ cations but is severely damaged with other cations. The procedures reported here are not only efficient to introduce a large amount of cations in mesoporous materials, but they can also be used to remove completely surfactant molecules from MCM-41 under mild conditions.

Supercritical Fluid Extraction of a Nonionic Surfactant Template from SBA-15 Materials and Consequences on the Porous Structure

SBA-15 mesoporous materials were synthesized using the method reported by Zhao et al. Surfactant was removed from as-made materials by means of different techniques: thermal treatment under air atmosphere, solvent washing at different temperatures, and supercritical CO2 extraction in the presence and absence of cosolvents. The structure of resulting materials was characterized using conventional techniques: nitrogen and argon adsorption measurements, powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), infrared spectroscopy (FT-IR), and 29Si MAS NMR. The efficiency of surfactant removal using CO2 under supercritical conditions is similar to that obtained by means of solvent washing under reflux but shows some improvements in the presence of cosolvents. Mesoscopic properties of mild-temperature solvent extracted SBA-15 materials depend on the efficiency of the surfactant removal and the use of supercritical CO2 as solvent. Likewise, the size and volume of the complementary microporosity dete...

Infrared Investigation of Fluoride Occluded in Double Four-Member Rings in Zeolites

Three infrared absorption bands related to fluoride occluded within double four-member ring units of zeolites exist: one corresponds to fluoride itself and the other two are localized vibrations of the cage appearing at adifferent wavenumber depending on the presence or absence of fluoride within the cage. This is demonstrated by comparison of the vibrational spectra of a series of as-made and calcined octadecasil materials containing varying degrees of fluoride and aluminum and is further supported by ab initio quantum chemical calculations.

Studies of the Synthesis of SSZ-25 Zeolite in a “Mixed-Template” System

The synthesis of aluminosilicate zeolite, SSZ-25, is described using a two-component organic guest molecule strategy. This method has been recently described by us and is quite effective in crystallizing a variety of aluminosilicate zeolites with reduction in template cost. In this instance the original organic guest component used in the discovery of SSZ-25, N′,N′,N′-trimethyl-2-adamantammonium cation, is used in conjunction with a variety of smaller amines. The amine is the major organic component in the synthesis and the quaternary ammonium compound, while a minor component, is essential for structure direction to the desired zeolite product. Studies here show that the adamantyl component is preferentially taken up during crystal growth. Our studies showed that the use of the multi-organic component approach to synthesis resulted in a faster and cheaper route to crystallizing SSZ-25. The SSZ-25 has been described in the literature as having the MWW topology and as such has two different channel systems accessed by 10-ring openings; the channel systems do not intersect. One set of 10-rings open into large cavities. In this study we used NMR spectroscopy to examine the as-made materials and to see if the two organic components are used in different capacities in the synthesis, probing if the admantyl component is only found in these larger cavities. The crystalline products could be affected by solvent extraction with polar solvents like dimethylformamide; the X-ray powder patterns and elemental analyses changed with solvent treatment. The solvent-treated crystals were studied by NMR spectroscopy, elemental analysis, and argon adsorption. The two-component organic guest approach was also found to be quite flexible not only with regard to the amine, but also towards the quaternary ammonium compounds. Non-adamantyl polycyclic templates could be used, and polar but non-quaternized adamantyl derivatives also succeeded in directing towards SSZ-25 formation. On the other hand, in one instance it was shown that the two-component approach favors SSZ-25 as the kinetic product but at longer run times Ostwald ripening was observed, leading to the disappearance of SSZ-25 and formation of ferrierite zeolite and quartz.

Studies of the synthesis of SSZ-25 zeolite in a "mixed-template" system.

The synthesis of aluminosilicate zeolite, SSZ-25, is described using a two-component organic guest molecule strategy. This method has been recently described by us and is quite effective in crystallizing a variety of aluminosilicate zeolites with reduction in template cost. In this instance the original organic guest component used in the discovery of SSZ-25, N',N',N'-trimethyl-2-adamantammonium cation, is used in conjunction with a variety of smaller amines. The amine is the major organic component in the synthesis and the quaternary ammonium compound, while a minor component, is essential for structure direction to the desired zeolite product. Studies here show that the adamantyl component is preferentially taken up during crystal growth. Our studies showed that the use of the multi-organic component approach to synthesis resulted in a faster and cheaper route to crystallizing SSZ-25. The SSZ-25 has been described in the literature as having the MWW topology and as such has two different channel systems accessed by 10-ring openings; the channel systems do not intersect. One set of 10-rings open into large cavities. In this study we used NMR spectroscopy to examine the as-made materials and to see if the two organic components are used in different capacities in the synthesis, probing if the admantyl component is only found in these larger cavities. The crystalline products could be affected by solvent extraction with polar solvents like dimethylformamide: the X-ray powder patterns and elemental analyses changed with solvent treatment. The solvent-treated crystals were studied by NMR spectroscopy, elemental analysis, and argon adsorption. The two-component organic guest approach was also found to be quite flexible not only with regard to the amine, but also towards the quaternary ammonium compounds. Non-adamantyl polycyclic templates could be used, and polar but non-quaternized adamantyl derivatives also succeeded in directing towards SSZ-25 formation. On the other hand. in one instance it was shown that the two-component approach favors SSZ-25 as the kinetic product but at longer run times Ostwald ripening was observed, leading to the disappearance of SSZ-25 and formation of ferrierite zeolite and quartz.

Synthesis and assembly of BaWO4 nanorods

Uniform, large aspect-ratio, monocrystalline BaWO4 nanorods were synthesized using a reversed micelle templating method; novel nanorod superstructures were observed in both as-made materials and Langmuir–Blodgett monolayer assemblies.

Synthesis and characterization of cobalt-containing hydrated aluminophosphate molecular sieves CoAPO4-H3

Pure, highly crystalline AlPO4-H3 (structure type APC) and cobalt-substituted materials CoAPO4-H3 have been synthesized hydrothermally in the presence of dipropylamine. As-made materials are free from organic molecules, which indicates that the amine does not act as a templating molecule but rather as a pH moderator. Under the synthesis conditions, cobalt could substitute for framework aluminum up to an extent of approximately two cobalt atoms per unit cell. Spectroscopic characterization of the samples showed that cobalt substitutes predominantly for four-coordinated aluminum species; i.e., in the 4.82 layers of the structure. The framework charge defect associated with the presence of Co2+ is compensated by protons, and the materials contain essentially Brönsted acid sites. As for pure AlPO4-H3, CoAPO4-H3 materials undergo structure modifications at elevated temperatures, thus leading to CoAPO4-C, CoAPO4-D and CoAPO4-H6. These transformations have been followed by various spectroscopic techniques, including 27Al two-dimensional–five-quantum magic-angle spinning nuclear magnetic resonance spectroscopy, and occur without any modification of the cobalt coordination and the oxidation state.

Investigation of structural iron species in Fe-mesoporous silicas by spectroscopic techniques

A series of iron-containing mesoporous silicas (Fe-HMS) with Fe contents up to 5.7 wt.% have been prepared at room temperature using the neutral hexadecylamine as surfactant molecule. Characterization of these materials by a variety of spectroscopic techniques, including EXAFS, EPR, Mossbauer and UV–Vis spectroscopies, suggests that Fe species are tetrahedrally coordinated in as-made materials. Tetrahedra are highly distorted with two long and two short Fe–O bond distances due to hydrogen-bonding type interactions with the organic molecules. Removal of the amines by solvent extraction does not modify the chemical environment around the Fe species. Subsequent calcination leads to mainly 3-coordinated Fe species, readily converting to 4-coordinated species in the presence of water. The solvent extraction appears to be a key step to prevent the formation of extraframework iron oxide species, which are formed in large amount when the surfactant is directly removed from as-made materials by calcination in air at high temperature.

Simple Preparation of Carbon-coated Iron Particles Having High Iron Content.

Carbon-coated iron materials were synthesized by a pyrolysis method using FeCp2 and Fe(CO)5/di-n-propylamine (DNPA) as starting materials. It was found that the Cp group was pyrolyzed to form carbon structure, whereas almost all of the CO group disappeared during pyrolysis and only DNPA could be an origin of the carbons. As a result, in the FeCp2-derived particles, the carbon yield was as high as 92.7% and iron content was 32.5%. In the Fe(CO)5/DNPA-derived particles, while the iron content was as high as 87%, the yield was only 16.2%. The structural and magnetic properties of these as-made materials suggest that α-Fe particles (5-20nm), possibly forming the magnetic field, were covered with thick carbon layer having external particle size of ca. 0.5-1μm. There were no iron species such as γ-Fe, FeC and Fe2O3 formed in the pyrolysis conditions employed.